The combination of chemical separation and analysis has long been recognized as invaluable to the analytical chemist in identifying chemicals at extremely low concentrations in complex matrices. For example, a drug and its metabolites can be effectively separated from blood plasma, using gas chromatography, and thereafter identified by the chemical fragments detected by mass spectrometry (see J. Chamberlain, The Analysis of Drugs in Biological Fluids, CRC Press, Boca Raton, 995, 2nd ed. chap. 6 and 7).
More recently, the combination of liquid chromatography, or flow injection analysis, with surface-enhanced Raman spectroscopy (SERS) has been investigated for such applications (see J-M. L. Sequaris and E. Koglin, Anal. Chem., 59,525 (1987); R. D. Freeman, R. M. Hanmaker, C. E. Meloan, and W. G. Fateley, Appl. Spectrosc., 42,456-460 (1988); F. Ni, R. Sheng and T. M. Cotton, Anal. Chem., 62, 1958(1990); G. T. Taylor, S. K. Shanna and K. Mohanan, Appl. Spectrosc., 44,635 (1990); R. Sheng, F. Ni and T. M. Cotton, Anal. Chem., 63,437 (1991); N. J. Pothier and R. K. Force. Appl. Spectrosc., 46, 147 (1992); L. M. Cabalin, A. Ruperez and J. J. Laserna, Talanta, 40, 1741 (1993); K. T. Carron and B. J. Kennedy, Anal. Chem., 67. 3353 (1995); L. M. Cabalin, A. Ruperez and J. J. Laserna, Anal. Chim. Acta, 318, 203(1996); N. J. Szabo and I. D. Winefordner, Appl. Spectrosc., 51.965 (1997); B. J. Kennedy, R. Milofsky and K. T. Carron, Anal. Chem., 69, 4708 (1997); and W. F. Nirode, G. L. Devault, M. J. Sepaniak and R. O. Cole, Anal. Chem., 72, 1866(2000)). Advantages of this combination of techniques include minimal sample preparation requirements, unrestricted use of water in the mobile phase, high chemical specificity through abundant molecular vibrational information, and extreme sensitivity, as demonstrated by the detection of single molecules. (See K. Kneipp, Y. Wang, R. R. Dasari and M. S. Feld, Appl. Spectrosc., 49,780(1995); and S. Nie and S. R. Emory, Science, 275, 1102 (1997)).
Previous research has employed primarily the three most common methods of generating SERS; i.e., using roughened silver or gold electrodes, using silver or gold-coated substrates, and using silver or gold colloids for detecting separated analytes. The lattermost method has gained the greatest amount of attention, since colloids can be prepared easily and inexpensively, and mixing of the colloids with the chromatographic column effluent, using flow injection, is reproducible. Care must be taken however to control aggregation of the colloids so that the amount of Raman signal enhancement is maintained. Also, a range of experimental variables, such as analyte concentration and pH, can strongly influence aggregation and, to some extent, limit applications; the choice of mobile phase is similarly limited by the need to maintain colloid integrity.
Recently, as described by Farquharson et al. in copending and commonly owned U.S. application Ser. No. 09/704,818 (published as International Publication No. WO 01/33189 A2, dated 10 May 2001), the entire specification of which is hereby incorporated by reference thereto, sol-gels have been developed to trap silver or gold particles as an improved method of generating plasmons for SERS (see also S. Farquharson, P. Maksymiuk, K. Ong and S. D. Christesen, SPIE, 4577, 166(2002); F. Akbarian, B. S. Dunn and J. I. Zink, J. Chem. Phys., 99, 3892 (1995); T. Murphy, H. Schmidt and H. D. Kronfeldt. SPIE, 3105, 40 (1997); and Y. Lee, S. Dai and J. Young, J. Raman Spectrosc. 28, 635 (1997)). It is appreciated that, once the sol-gel has formed, the particle size and aggregation of the metal dopant are stabilized, albeit changes in pH may still result in variable Raman signal intensities, such as in the case of weak acids and bases, where the relative concentrations of the ionized and unionized forms may be influenced. Also, it has been shown that many of the common solvents, such as acetone, methanol, and water, can be used equally with these metal-doped sol-gels in generating SERS of analytes.
In accordance with other recent developments, moreover, sol-gels have been used as the stationary phase in columns for liquid- and gas-phase chromatography, affording advantages in both the preparation of columns and also in their performance. The sol-gel approach enables deactivation, coating, and immobilization to be combined as a single step, while the sol-gels have shown reduced tailing, improved separation, and broader application to solvents and analytes.
Microchip devices have also been employed for effecting chemical separations (see Jacobson, S. C., Hergenröder, R., Koutny, L. B., & Ramsey, J. M. “High-Speed Separations on a Microchip,” Anal. Chem., 66, 1114-1118 (1994); Jacobson, S. C., Hergenröder, R., Koutny, L. B., Warmack, R. J., & Ramsey, J. M. “Effects of Injection Schemes and Column Geometry on the Performance of Microchip Electrophoreis Devices,” Anal. Chem., 66, 1107-1113 (1994); Jacobson, S. C. Hergenröder, R., Koutny, L. B. & Ramsey, J. M. “Open Channel Electro-chromatography on a Microchip,” Anal. Chem., 66, 2369-2373 (1994); and Moore, Jr., A. W., Jacobson, S. C. & Ramsey, J. M. “Microchip Separations of Neutral Species via Micellar Electrokinetic Capillary Chromatography,” Anal. Chem., 67, 4184-4189 (1995)).